/*
自己选择的路 ,跪着也要走完。朋友们 , 虽然这个世界日益浮躁起来,只
要能够为了当时纯粹的梦想和感动坚持努力下去 , 不管其它人怎么样,我
们也能够保持自己的本色走下去。
To the world , you will be a person , but to a person , you
will be the world .                               ——AKPower
*/
#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <vector>
#include <map>
#include <queue>
#include <cstdio>
#include <string>
#include <stack>
#include <set>
#define IOS ios::sync_with_stdio(false), cin.tie(0)
using namespace std;
typedef long long ll;
const ll maxn = 100000;
vector<pair<ll, ll>> v;
queue<ll> p;
pair<ll, ll> a[maxn];
int main()
{
    IOS;
    ll n, x, y;
    cin >> n;
    for (ll i = 0; i < n; i++)
    {
        cin >> x >> y;
        a[i].first = x;
        a[i].second = y;
        if (x == 1)
            p.push(i);
    }
    while (!p.empty())
    {
        ll temp = p.front();
        p.pop();
        if (a[temp].first == 0)
            continue;
        ll ty = a[temp].second;
        v.push_back(make_pair(temp, ty));
        a[temp].first--;
        a[temp].second ^= ty;
        a[ty].first--;
        a[ty].second ^= temp;
        if (a[ty].first == 1)
        {
            p.push(ty);
        }
    }
    ll len = v.size();
    cout << len << endl;
    for (ll i = 0; i < len; i++)
    {
        cout << v[i].first << " " << v[i].second << endl;
    }
    getchar();
    getchar();
    return 0;
}
